Figure 5-2.-Types of projections.(fig. 5-2) is further classified into subtypesaccording to the direction of its projectionlines relative to the plane of projection. If theprojection lines, in addition to being parallel toeach other, are perpendicular (normal) to theplane of projection, the result is an orthographicprojection. If they are parallel to each other butoblique to the plane of projection, the result isan oblique projection.To better understand the theory of projection,you must become familiar with certain elementsthat are common to each type of projection. Someof these elements are defined below.The POINT OF SIGHT (or STATIONPOINT) is the position of the observer inrelation to the object and the plane of projection(fig. 5-2). It is from this point that the view ofthe object is taken. The point of sight is changedto give different views of the same object; hence,there must be a different point of sight for eachview. Imagine yourself looking first at the frontof an object, then down at the top, and then atthe right or left side, as the case may be. Eachadditional view requires a new point of sight.The observer views the features of the objectthrough an imaginary PLANE OF PROJEC-TION (or IMAGE PLANE). In parallel projec-tion, this theoretical transparent plane is placedbetween the point of sight and the object, asshown in figure 5-2. For perspective pictorials, itFigure 5-3.-Basic orthographic projection.5-2